Freeze drying apparatus and method



Jan. 25, 1966 W- H HAMILTON 3,230,633

FREEZE DRYING APPARATUS AND METHOD Filed 001'.. 27, 1961 2 Sheets-Sheet l INVENTOR 74 75 w|LL|AM H HAMILroN ATTORNEY MOISTURE LOSS Jan. 25,

W. H. HAMILTON FREEZE DRYING APPARATUS AND METHOD Filed 001;. 27, 1961 2 Sheets-Sheet 2 ATTORNEY United States Patent O 3,230,633 FREEZE DRYING APPARATUS AND METHGD William H. Hamilton, Philadelphia, Pa., assignor, by mesne assignments, to Pennsalt Chemicals Corporation, Philadelphia, Pa., a corporation of Pennsylvania Filed Oct. 27, 1961, Ser. No. 148,126 Claims. (Cl. 34-15) This invention relates to vacuum drying apparatus and a method of drying articles under vacuum in a manner whereby the drying time is reduced and the avor or nutrient characteristics of the articles are improved.

Heretofore, it has been conventional in the art to dry articles by applying heat thereto while the articles are disposed in an evacuated chamber. Thus, drying apparatus suitable for freeze drying articles includes a housing having heated shelves and means for evacuating the housing. As a general rule, articles of food and the like are generally disposed within a chamber which has been evacuated to a pressure of approximately 100 to 3,000 microns. For example, see Patent 2,388,917 which states that the freeze drying process preferably takes place in an evacuated chamber wherein the pressure is 100 microns or less.

The housing of the drying apparatus is evacuated since it is well known that the temperature required to effect drying is a function of pressure. Decreasing the pressure lowers the temperature required to effect drying. Thus, it has been proven in a laboratory that it is possible to boil water at room temperature when the chamber wit-hin which the Water is disposed is at a sufficiently low pressure.

The drying apparatus and method of the present invention is of the general type described above wherein drying takes place in an evacuated chamber with the application of heat to the articles. I have discovered that the drying cycle may be substantially reduced by increasing the pressure within the chamber after a predetermined portion of the cycle has been completed. Thus, I have discovered that the remainder of the 4drying cycle may be substantially reduced by increasing the pressure in the chamber when the rate of moisture loss starts to decrease. Preferably, the increased pressure is effected by passing a gas through the housing.

By introducing gas into the housing to increase the pressure in the chamber, and by operating the vacuum pump in a throttled condition, the articles will be dried by convection as well as radiation during the remainder of the drying cycle. While the gas increases the vapor pressure on the article, this effect is offset by the increase in the vapor loss due to convection drying. Thus, articles dry faster when dried with the apparatus and method of the present invention.

It is an object of the present invention to provide a drying apparatus and method which shortens the drying cycle without adversely effecting the quality and characteristics of the product.

lt is another object of the present invention to provide a vacuum drying apparatus and method which shortens the drying cycle.

It is another object of the present invention to provide a novel freeze drying apparatus which enables articles to be freeze dried in a shorter period of time.

It is another object of the present invention to provide ICC a vacuum drying apparatus and method which shortens the drying cycle by increasing the pressure within the drying chamber after a predetermined portion of the cycle has been completed.

It is still another object of the present invention to provide a vacuum drying apparatus and method which shortens the drying cycle by increasing the pressure Within the drying chamber and decreasing the shelf temperature after a predetermined portion of the cycle has been completed.

It is still another object of the present invention to provide a novel vacuum drying apparatus and method which shortens the drying cycle by flowing a gas through the drying chamber after a predetermined portion of the drying cycle has been completed so as to increase the pressure in said chamber.

It is a further object of the present invention to provide a freeze drying apparatus and method which shortens the drying cycle by heating the articles by convection after a predetermined period of the cycle hasl been completed.

Other objects will appear hereinafter.

For the purpose of illustrating the invention 'there is shown in the drawings forms which are presently preferred; it being understood, however, that this invention is not limited to the precise arrangements and instrumentalities shown.

FIGRE 1 is a schematic diagram of the drying apparatus of the present invention.

FIGRE 2 is a schematic diagram of another embodimen of the drying apparatus of the present invention.

FIGURE 3 is a graph illustrating vapor loss versus' drying time with temperature and pressure superimposedv thereon.

Referring to the drawing in detail, wherein like numerals indicate like elements, there is shown in FIG- URE 1 one embodiment of the pre-sent invention designated generally as 10.

The apparatus 10 comprises a housing 12 having a drying chamber therein. The drying chamber within the housing 12 may be evacuated through the conduit 14. The conduit 14 is in open communication with the drying chamber and a condenser 18. A valve 16 is disposed in the conduit 14 between the drying chamber and the condenser 18. A vacuum pump 22 has its inlet side in communication with the outlet side of the condenser 18. A solenoid operated valve 20 is disposed intermediate the condenser 18 and pump 22 for throttling the pump 22 as will be made clear hereinafter.

A shelf 24 is supported within the drying chamber of the housing 12. While only one shelf 24 is illustrated, it will be appreciated that a larger number of shelves horizontally spaced one above the other are generally provided in commercial apparatus. The shelf 24 may be hollow with conduits connected to the interior thereof for conveying a heating fluid such as steam. Preferably, the shelf 24 is an electrically heated type since electrically heated shelves provide almost instantaneous heat, may be extremely accurately controlled as to shelf temperature, and are thinner than hollow iluid heated shelves. Thus, electrically heated shelves permit a greater number of trays of articles to be supported within the housing 12.

The particular structure of the shelf 24, per se, forms no part of the present invention. For purposes of illustration, the shelf 24 may comprise a rigid heat resistant electrically non-conductive layer of material having an electrically conductive coating thereon. Preferably, such electrically conductive coating is on opposite surfaces thereof so that the shelf 24 radiates heat upwardly and downwardly. The electrically conductive coating will have a film of electrically non-conductive material such as lacquer or the like thereover to prevent short circuiting and ease of handling. The electrically conductive coating will be connected to terminals 30 and 32 by wires 26 and 28. Wire 28 is preferably provided with a rheostat 34.

A tray 38 for the articles to be dried may be supported within the drying chamber by separate supports or may be supported by the shelf 24. In the latter event, a spacer 36 of mesh-type electrically non-conductive material-will be provided so that the articles within the trayV 38 will be heated by radiant heat from above and below. In order to provide for escape of vapor, the bottom wall of the tray 38 may be perforated.

In use, the tray 38 will be substantially filled with articles to be dried such as article 40. The articles should be of substantially the same size, shape, and the same material. By the same material is meant that all of the articles 40 will be shrimp, for example, as compared with a mixture of shrimp, bone chips, coffee extract, etc.

One of the articles 40'will have a resistor embedded therein. The resistor will be connected to a weight recorderandcontroller 48 by wires 42 and 44. An amplifier 46 will be connected to one of said wires, such as wire 44.y The weight recorder and controller 48 will be connected-to a source of potential having terminals S and 52.

The weight recorder and controller 4S may be connected -to a temperature recorder and controller 54 by wires as illustrated.- The temperature recorder and controller 54 may be connected to the rheostat 34 by wires 56 and 58., A conduit 60 has one end in communication with the drying chamber in the housing 12. The other end of theconduit 6.0 is vconnected to a source of gas 61 which may be atank of pressurized inert gas such as nitrogen.

A solenoid operated pressure regulator type valve 62 is disposed within the conduit 60. The valve 62 is normally closed and will be opened at a predetermined point as will be made clear hereinafter. The solenoid operator for the valve 62 isJconnected to the weight recorder and controller 48 by wires 64 and 66. The solenoid operator for the throttling valve 20 is connected to the weight recorder and controller 48 by Wires 68 and '70.

The operation of the-apparatus illustrated in FIGURE 1 will be explained in conjunction with the graph illustrated in FIGURE 3. The graph iillustrated in FIGURE 3 is based on test results wherein cooked whole shrimp was freeze dried with the shrimp being placed on end in a wire tray having anl area of approximately .5 square feet. The shrimp was stacked in a layer of approximately two inches thick. The original weight of the shrimp was 1084 grams.

The frozen shrimp were disposed within trays and supported bythe shelves within the drying chamber in spaced relation therewith so that .the shrimp were heated by radiant heat from aboveand below. The shelf temperature at the beginning of the yheat cycle was approximately 150 C. and the pressure within the drying chamber was maintained atapproximately 2400 microns. As indicated by the graph of the product temperature, the temperature of the product atthe beginning of the cycle was below freezing. As seen from the graph of the moisture loss, the largest portion of the moisture in the shrimp is sublimed during the first half of the cycle. Thus, ninety percent of the-moisture is removed from the shrimp in the rst six and one-half hours. During a normal cycle, the next tive percent ofthe moisture would require approximately nine and one-half hours to remove the same. At approximately this point in the cycle, the pressure within the chamber was doubled from 2400 microns to approximately 4800 microns and maintained at said latter mentioned pressure for the remainder of the cycle. Likewise, the shelf temperature was reduced from approximately C. to approximately 65 C. At this point, it will be noted that the product temperature decreased substantially and the rate of moisture removal increased. Accordingly, the above mentioned changes in the drying cycle enable the shrimp to be dried to a point where only five percent of the moisture remained therein within nine and one-half hours as compared with sixteenth hours during a normal cycle.

It will be appreciated that as the moisture remaining within the shrimp constantly changes, the electrical` resistance likewise changes. When the electrical resistance reaches a predetermined. point, the resistance coupled to the weight recorder and controller 48 through the wires 42 and 44 enables the controller 48 to open valve 62, vary the rheostat 34 through the temperature controller 54, and throttle the valve 20. The opening of the valve 62 enables a gas from source 61 to ow into the drying chamber. The entry of gas into the drying chamber increases the pressure therein. Said gas introduced into the drying chamber is removed at the same rate that it enters the-chamber by the pump 22. Thus, gas is flowing through the drying chamber-and heating the shrimp by convection. The shelf temperature need not be reduced. However, such reduction in the shelf temperature prevents overheating of the shrimp, and prevents boiling oif fats and other :nutrients in the article being freeze dried.

It will be appreciated that merely throttling the Valve 20 willincrease the pressure within the drying chamber. However, such throttling of the valve 20 is preferably performed in conjunction with the introduction of gas into the drying chamber.

The above mentioned process involving shrimp as the article being freeze ldried can be accomplishedby gradually increasing the pressure and/or gradually decreasingthe shelf temperature. Shrimp as Well `as meat and otherproducts are provided with fibrous tissues which can be processed by increasing 'the pressure as the moisture loss rate begins to decrease. For example, the pressure within the drying chamberwould have been increased gradually from 2400 microns to 4800 microns during the time interval between four and one-half hours and six and one-half hours after-initiation of the cycle. When freezing drying coffee extract, it is necessary to postpone the increase in the `pressure .until the` coffee extract structure has set. That is, the increase in pressure must be postponed until the structure of kthe coffee extract has been formed to a point so that the structure would not change if the remaining moisture therein were to melt.

Heretofore, it was considered inadvisable to increase the pressure within the drying chamber during the Adrying cycle since it was thought that the. same would increase the drying cycle rather than shorten the same. In this regard, it must be remembered that the article being freeze dried has attained a point wherein it is largely a porous mass which functions as a good heat insulator. The phenomena of substantially shortening the drying cycle whereby ninety-tive percent of the moisture was removed in nine and one-half hours -as compared lwith sixteen hours was therefore completely unexpected. It will be appreciated that the desirability to remove ninetyfive percent of the moisture from the `articles being freeze dried is an arbitrary gure and varies with the particular articles being freeze dried and the ease with Iwhich the last few percent of moisture may be removed.

Empirical tables have been developed heretofore to indicate the pressure, temperature, and length of drying cycle for substantially all types of artifclesadapted to be freeze dried. Accordingly, resort may be had to such` tables to ascertain the amount of moist-ure which has been removed at any time period during the freeze drying cycle. It is possible to utilize such tables to control the freeze drying cycle so that the pressure within the chamber may .be increased in accordance with the present invention. Thus, there is Ashown in FIGURE 2 a schematic diagram of another embodiment of the present invention designated as The apparatus 10 is identical with the apparatus 10 except as will be made clear hereinafter. An automatic timing mechanism 72 is connected to a source of electrical potential having terminals 74 and 76. The timing mechanism 72 is connected to the solenoid operator for the valve 20 by wires 78 and 80. The timing mechanism 72 is connected tothe solenoid operator for the valve 62 by wires 82 Aand 84. The timing mechanism 72 is connected to the rheostat 34 by wires 86 and 88.

The timing mechanism is of a type which may be manually preset so that valve 62 may be opened, valve 20 throttles, and rheostat 34 varied after such predetermined period of time has expired. Thus, the pressure within the chamber 12 may be increased and the pump 22 throttled at a predetermined time interval after the initiation of the freeze drying process as determined from the above mentioned empirical tables. Otherwise, the apparatus 10 is identical with the apparatus 10. The -apparatus 10 and 10' are designed to terminate the drying cycle upon completion thereof and actuate an audible or visual signal indicating completion of the cycle. The weight recorder and controller 48 and temperature recorder and controller 54, per se, form no part of the present invention since the same are commercially available equipment. Like-wise, the timing mechanism 72 is commercially available equipment .and per se forms no part of the present invention.

In accordance with the illustration in FIGURE 3, the .pressure within the chamber was doubled from 2400 microns to 4800 microns. It is within the scope of the present invention to increase the pressure within the chamber up to atmospheric pressure. It is within the scope of the present invention to provide a timer switch for the wire 64 or wi-re 82 so that a gas is periodically injected into the drying chamber. In this manner, it would not be necessary to throttle the valve 20 for the pump 22. It should be noted that it is witlLn the scope of the present invention to maintain the shelf temperature constant and thereby eliminate the decrease in the shelf temperature as the pressure Within the drying chamber is increased. It is preferable to decrease the shelf temperature so as -to 4avoid overheating the articles and avoid distilling fats and other nutrients in the articles thereby improving the taste of the articles. It will be appreciated by those skilled in the art that the embodiments illustrated in FIGURES 1 and 2 merely illustrative of a wide variety of devices which may be utilized to increase the pressure within the drying chamber at a predetermined point in the cycle so that the balance of the cycle results in the articles being dried by convection and radiant heat.

It is within the scope of the present invention to alternately raise and lower the pressure within the chamber. As the pressure is increased, the product temperature increases. When the pressure is reduced the product remains at this temperature thereby flashing olf moisture. In this manner, length of the cycle is shortened to an additional extent.

As pointed out above, periodic variation of the pressure may be accomplished by cyclically opening and closing valve 62. Referring to FIGURE 3, cyclic variation of the pressure may occur as follows. After six hours of drying time, increase the pressure to 4800 microns for several minutes, then decrease the pressure to 3600 microns for several minutes, then increase the pressure to 4800 microns, etc. It will be appreciated that the pressures can be pulsed back and forth with substantially no dwell period or the dwell period may be increased since the length of the dwell period may vary depending on 'the type of product being freeze dried.

The present invention may be embodied in other specific forms without departing from the spirit or essential attributes thereof and, accordingly, reference should be made to the appended claims, rather than to .the foregoing specification as indicating the scope of the mvention.

I claim:

1. Apparatus for freeze drying articles comprising a housing, a vacuum pump for evacuating said housing to a rst pressure, said pump being connected to said housing by a pumpingconduit, means supporting shelves in said housing, means for selectively elevatingthe temperature of said shelves, and selectively operable valve means for increasing the pressure in said housing to a second pressure which is between said first pressure and atmospheric pressure by flowing a gas through said housing, a condenser in said pumping conduit between said pump and said housing, and a throttling valve between said pump and said condenser, whereby said gas may be removed frorn said housing with said pump being in a throttled condition.

2. Apparatus in accordance with claim 1 including a timer mechanism structurally interconnected with said selectively operable valve means and throttling valve so that said throttling valve may be restricted and said selectively operable valve means may be operated after a predetermined set period of time.

3. Apparatus for drying articles comprising a housing, means connected to said housing for evacuating a chamber within said housing to a first pressure, means within said housing for heating articles adapted to be dried within said chamber, and means for increasing the pressure in said chamber to a second pressure which is greater than said iirst mentioned pressure by circulating a gas through said housing after a predetermined period during the drying cycle so that the articles adapted to be disposed within said chamber are heated by convection for the balance of the drying cycle, and means for decreasing the temperature of the heating means when the pressure in said chamber is increased.

4. A method of drying articles comprising placing articles in a chamber, evacuating said chamber to a irst pressure while elevating the temperature in said chamber, maintaining said chamber at said temperature and pressure for a portion of the drying cycle until a substantial amount of moisture has been removed from said articles, then increasing the press-ure in said chamber to a pressure above said iirst pressure while decreasing the temperature of the article, and completing the drying cycle at a pressure above said irst pressure and at a temperature below said first-mentioned temperature.

5. A method in accordance with claim 4 wherein the pressure is increased by introducing a gas into said chamber with said gas being removed by the means evacuatin g said chamber.

6. A method of freeze drying articles comprising placing `trays of frozen articles in a chamber, evacuating said chamber to a first pressure while subjecting said articles to heat sufficient to sublime ice in said articles, maintaining said pressure in said chamber and subjectingsaid articles to heat until the moisture removal rate starts to decrease, then drying said articles by convection for the balance of the drying cycle while said articles are disposed in the evacuated chamber and subjected to a .temperature which is lower than the temperature to which said articles are subjected by said step of subjecting the articles to heat suicient to sublim-e ice in said articles.

7. A method in accordance with claim 6 wherein said articles are sublimed at said -rst pressure until the articles have attained their set structure.

8. A method in accordance with claim 4 including repeatedly increasing and decreasing` the pressure in said chamber during the drying cycle after a substantial por# tion of the moisture has been removed from the articles.

9. A` method in accordance with claim 6 wherein the drying of said articles by convection is accomplished by introducing gas into said chamber so as to increase the pressure of said chamber above said first pressure, removing said gas by `the means evacuat'in'g Saidchamber, and reducing the temperature of the heat in said chamber simultaneously with the increasing of the pressure within said chamber.

10. A method in accordance with claim 6 wherein the moisture in said articles is sublimed at said first pressure until .the articles have attained their said structure.

Reinhardt 34'-15 Clark 34-15 Moore 34-15 Irvvin 97-127 Freund 34-15 Levinson 34-5 X Parkinson 34-5 X Neumann 34-5 Copson 345 Marsh 34-5 ROBERT A. OLEARY, Primary Examiner. NORMAN YUDKOFF, Examiner.- 

4. A METHOD OF DRYING ARTICLES COMPRISING PLACING ARTICLES IN A CHAMBER, EVACUATING SAID CHAMBER TO A FIRST PRESSURE WHILE ELEVATING THE TEMPERATURE IN SAID CHAMBER, MAINTAINING SAID CHAMBER AT SAID TEMPERATURE AND PRESSURE FOR A PORTION OF THE DRYING CYCLE UNTIL A SUBSTANTIAL AMOUNT OF MOISTURE HAS BEEN REMOVED FROM SAID ARTICLES, THEN INCREASING THE PRESSURE IN SAID CHAMBER TO A PRESSURE ABOVE SAID FIRST PRESSURE WHILE DECREASING THE TEMPERATURE OF THE ARTICLE, AND COMPLETING THE DRYING CYCLE AT A PRESSURE ABOVE SAID FIRST PRESSURE AND AT A TEMPERATURE BELOW SAID FIRST-MENTIONED TEMPERATURE. 